Gene therapy is being considered in the treatment of several acquired and inherited diseases. Previous studies have focused primarily on bone marrow cells as targets for gene transfer, however, many diseases will require genetic modification of other somatic tissues such as the central nervous system or the liver. We plan to develop methods for directing gene replacement therapies to the liver. Familial hypercholesterolemia (FH), an autosomal dominant disorder in humans caused by a deficiency in the receptor for LDL (LDL receptor), will be used as a model for the development of liver-directed gene therapies. Features of this genetic disease that make it a particularly useful model include: 1) the biochemical, pathophysiological, and clinical aspects of FH have been well described and the corresponding normal gene is cloned and available, 2) phenotypic correction of the metabolic abnormalities associated with FH will probably require gene transfer into the hepatocyte because the liver is the primary organ responsible for degradation of the LDL (via LDL receptor) and the only organ capable of excreting cholesterol (via bile acids and biliary cholesterol), 3) and authentic animal model for FH exists, the Watanbe Heritable Hyperlipidemic (WHHL) rabbit, and 4) no effective conventional therapy exists for homozygous deficient FH other than combined liver/heart transplantation. The WHHL rabbit will be used to develop two different types of gene replacement therapies. The ex vivo approach to gene therapy will involve harvesting hepatocytes from an LDL receptor-deficient animal, transfecting a functional LDL receptor gene into the hepatocytes, and transplanting the genetically modified cells back into the affected animals. A potentially more effective and less morbid approach to the genetic treatment of FH is to target the delivery of a functional LDL receptor gene to hepatocytes in vivo. Hepatocyte targeting methods will be based on interactions with the hepatocyte specific receptor, the asialoglycoprotein receptor. Stable expression of the transferred gene will be achieved through the development of vector systems that either integrate in genomic DNA or persist as extrachromosomal elements. The development of liver-directed gene therapies in the WHHL rabbit should greatly facilitate the eventual application of these technologies to the treatment of patients with FH or other liver specific disorders.